A power supply or power converter converts one form and voltage of electrical power to another desired form and voltage. AC-to-DC power supplies convert alternating current voltage, for example 115 or 230 volt alternating current (AC) supplied by a utility company, to a regulated direct current (DC) voltage. DC-to-DC power supplies convert DC voltage at one level, for example 400V, to another DC voltage, for example 12V.
A variety of different DC-to-DC power converter configurations are currently in use, most of which are variations of a buck converter, a boost converter, and a buck-boost converter. Some variations of buck converters, referred to as isolated buck-type converters, include an isolating transformer. Some versions of isolated buck-type converters include the push-pull converter, the forward converter, the half-bridge converter, and the full-bridge converter. Buck-type converters can either be duty-cycle-controlled switched converters, or they can be frequency-controlled resonant converters.
Each type of isolated buck-type converter can include various combinations of rectifiers and windings on the secondary side of the transformer. One typical variation is a full-bridge rectifier, which comprises 4 diodes configured to produce a same-polarity voltage output regardless of the polarity of the secondary winding voltage. A second typical variation comprises a center-tapped output such that the center tap is connected to a common point, and ends of each of the other two windings are connected to the anode of a diode. The cathodes of both diodes are connected to an output capacitor, and the other side of the output capacitor is attached to the center tap. Another variation is a current-doubler circuit. Still another variation is a split output in which one side of a center-tapped secondary charges a first output capacitor to a positive voltage and the other side of the center-tapped secondary charges a second output capacitor to a negative voltage. The two output capacitors are also connected to the center tap of the transformer.
The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power. Low-power-factor loads increase losses in a power distribution system and result in increased energy costs. Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor that is less than 1. Power factor correction attempts to adjust the power factor to unity (1.00).
AC-to-DC converters above approximately 75 W, as well as some specific applications below 75 W, require the converter to draw current from the AC line with a high power factor and low harmonic distortion. Most conventional methods to produce a power factor corrected power supply with isolated low voltage DC outputs include cascading converter stages.
The term “cascading converter stages” refers to the use of multiple power conversion stages such that the output of one converter stage is connected to the input of the subsequent stage. Each converter stage uses controlled semiconductors such as MOSFETs or IGBTs to control the voltage, current, and/or power at the output and/or input of the converter stage. So, for example, a full-wave passive rectifier bridge is not considered to be a converter stage. While cascaded converter stages may share control circuitry, house-keeping power supplies, or communication with each other, the power semiconductors and energy storage elements that form each converter stage perform a power conversion function that is primarily independent of any other converter stage. Typical examples of converter stages are isolated or non-isolated variants of a buck converter, a boost converter, a buck-boost converter, and a sepic converter.
AC-to-DC power conversion is typically accomplished with cascaded converters instead of with a single-stage converter. For example, many AC-to-DC converters use two primarily independent converter stages: a first converter stage steps the input rectified sinusoidal voltage up to a high-voltage bus, and a second converter stage steps the high-voltage bus down to a low-voltage bus as well as provides isolation. While it is common for these two converter stages to communicate with each other, and while the ripple noise and loading effects on one converter have some effect on the other, these two types of converter stages can operate primarily independently of each other.
Cascading converter stages typically results in low overall efficiency since the overall efficiency is influenced by each stage. For example, if the first stage has an efficiency of 93% and the second stage has an efficiency of 93%, the overall efficiency is about 86.5%. In some cases there are three or more cascaded converter stages. For example, AC-to-DC converters with multiple outputs, such as 12V, 5V, and 3.3V, can use two cascaded converter stages to produce 12V and 5V, and then use a third cascaded stage to produce a 3.3V output from the 5V output. If the efficiency of the first cascaded stage, for example a non-isolated boost PFC converter, is 93% and the efficiency of the second stage, for example an isolated full-bridge converter is 93%, and the efficiency of the third cascaded stage, for example a 5V to 3.3V non-isolated buck converter is 96%, then the overall efficiency at the 3.3V output is only 83%. The efficiency very quickly degrades as more converter stages are cascaded to arrive at the final output voltage.
Single-stage converters contain no intermediate DC bus. A single-stage isolated AC-to-DC converter typically uses a passive rectifier to converter the AC input to a DC voltage. In the case of single-stage PFC AC-to-DC converters, the DC voltage at the output of the passive rectifier is similar to a full-wave-rectified sinewave, for example the absolute value of a sinusoid. In the case of non-PFC AC-to-DC single-stage converters, the output from the passive rectifier is typically connected to a large bulk capacitor, which causes the voltage at the output of the rectifier to resemble a nearly constant voltage with a small amount of second-harmonic line-frequency ripple superimposed on top of the constant voltage.
A single-stage AC-to-DC converter uses a single isolated power conversion stage to convert voltage at the output from the rectifier to a voltage, which is electrically isolated from the AC input.
The term “isolation” refers to isolating the input voltage from the output voltage. In particular, isolating means there is no path for DC current between the power supply's input source and its output terminals or load. Isolation is achieved using a power transformer in series with the power flow from input to output. Isolation can be applied to the power converter as a whole, or to individual components within the power converter where the voltage input to the component is isolated from the voltage output from the component.
Conventional technologies typically use one of two methods to provide an isolated DC output and a high power factor input. The first method uses a boost converter (step-up converter) to produce a high voltage bus (typically 250 VDC to 400 VDC), which is then cascaded with an isolated buck-type converter to step the high voltage bus down to an isolated low voltage output. This technique is relatively expensive and not extremely efficient.
A first conventional high power-factor isolated converter from the first method described above includes a boost converter to produce the high power factor input. Boost converter power factor correction circuits are limited in configurations. Voltage-source boost converters cannot be configured to provide an isolated output so another converter stage is included to provide isolation. Furthermore, boost converters are limited in their ability to be configured for soft-switching and resonant switching techniques, so these boost converters may produce large amounts of EMI, high losses (if operating at high frequency), and they often include expensive boost diodes to avoid problems with large reverse recovery losses in their diodes. Soft-switching, which can be accomplished through zero-voltage switching or zero-current switching, uses circuit resonance to ensure that power transistors switch at or near a zero-voltage level or zero-current level. This reduces the stress of the transistor component and also reduces the high frequency energy that would otherwise be radiated as noise. Hard-switching is the simultaneous presence of voltage across the transistor and current through the transistor when the transistor turns on and when the transistor turns off. This condition results in power dissipation within the device.
FIG. 1A illustrates a block diagram of a first conventional power factor corrected isolated converter according to the first conventional method. An EMI filter 18 is typically coupled between an AC input source 16 and the rest of the converter to prevent noise from coupling back to the AC source. The EMI filter 18 is coupled to a full-wave diode rectifier bridge 20 configured to provide a rectified sinusoidal input voltage to the rest of the converter. A non-isolated boost converter 21 draws a nearly sinusoidal current from the AC input source 16 and charges a high voltage bulk capacitor to typically 250V to 400V, thereby generating a high-voltage bus. An isolated buck-type converter 22 and an isolation transformer 24 steps the high voltage bus down to an isolated low voltage output.
The non-isolated boost converter 21 in FIG. 1A is typically hard-switched. Furthermore, to overcome high switching losses in the boost converter diode, the boost converter 21 typically either uses a silicon carbide diode, which is relatively expensive, or additional parts are added to enable soft-switched transitions, which is also expensive, or the boost converter uses critical or discontinuous conduction mode, which is applicable primarily to low power levels due to the extremely high ripple currents generated at the input of the converter.
The isolated buck-type converter 22, such as a full-bridge converter, includes the isolation transformer 24 to generate an isolated secondary output voltage. The secondary output voltage is rectified and filtered by rectifiers 26 to generate a DC output voltage.
The second converter stage, the isolated buck-type converter, is needed because the boost PFC converter cannot easily produce isolation. The first converter stage, the boost PFC converter, is used because isolated buck-type topologies typically cannot be effectively used to provide both the power factor correction and the isolated regulated voltage. Isolated buck-type converters cannot be effectively used to provide both the PFC and the isolation because they are unable to draw current from the input when the input voltage scaled by the turns ratio of the isolation transformer drops below the output voltage. In order to design a traditional isolated buck-type converter to accomplish both PFC and isolation, a designer would have to make a choice between either not drawing current from the input line for much of the input sinewave, or designing the isolation transformer turns ratio such that the output voltage multiplied by the isolation transformer primary-to-secondary turns ratio is a very low voltage. In the case where current is not drawn from the input line for much of the input sinewave, the power factor is low and the converter does not adequately accomplish power factor correction. In the case where a high turns ratio is used to enable drawing current for much of the input sinewave, the converter efficiency is low because the converter must operate at low duty cycles (or far off-resonance for a resonant converter) for the majority of the input sinewave.
The second method of providing an isolated DC output and high power factor input is typically directed to lower power applications and uses a flyback converter to provide a single-stage isolated converter that draws a high power-factor. A flyback converter is an isolated buck-boost converter in which the inductor is combined with the transformer, thereby multiplying voltage ratios and providing an isolated voltage output.
FIG. 1B illustrates a block diagram of a second conventional power factor corrected isolated converter according to the second conventional method. The second conventional power converter is configured similarly as the first conventional power converter of FIG. 1A except that the non-isolated boost converter 21 and the isolated buck-type converter 22 of FIG. 1A are replaced by a flyback converter 122, and the isolation transformer 24 of FIG. 1A is replaced by an isolated flyback transformer 124. The flyback converter 122 is configured to draw current at low input voltages as well as to provide isolation.
The second conventional power converter shown in FIG. 1B has a number of problems. The isolated flyback transformer 124 only transfers power to the load while the main transistor switch of flyback converter 122 is in an off state. The core of the transformer is therefore poorly utilized. Furthermore, the flyback converter 122 is hard-switched, which leads to high switching losses and EMI generation. Generally, flyback converters have a low efficiency and are only cost effective at low power.
A tap switch is a semiconductor switch coupled to a secondary winding of a transformer and is used to effectively increase, or decrease, the functional turns ratio of the transformer. Tap switches are utilized in AC-to-AC applications in order to make small adjustments to a transformer output voltage in response to variations in the rms input voltage. Tap switches can also be utilized in DC-to-DC applications, for example in increasing hold-up time of the converter by boosting the output voltage when the bulk cap voltage decreases immediately following a power outage.